The present invention relates to an apparatus for detecting the failure of nuclear fuel.
Generally, the reactor core of a boiling water reactor is loaded with a multiplicity of fuel assemblies each of which is constituted by an upper and a lower tie plates, a channel box and a plurality of fuel rods which are supported at their both ends by the upper and the lower tie plates and are accommodated in the channel box.
In case of a failure of the fuel rod or rods of the fuel assembly mounted in the reactor core, fission products such as radioactive iodine, xenon, krypton and the like are discharged into the cooling water circulated through the reactor core. If the nuclear reactor operates continuously with its reactor core loaded with fuel assemblies including failed fuel rod or rods, the contamination in the reactor caused by the fission products will be serious so as to hinder the work for the protective maintenance of the nuclear reactor.
To avoid this, the fuel assemblies with which the reactor core is loaded are periodically checked to detect the failure of the fuel rods.
If a failure is detected, the fuel assembly containing the failed fuel rod is withdrawn from the reactor core, and a new fuel assembly is placed instead of the failed fuel assembly.
The specification of U.S. Pat. No. 4,147,587 discloses a method of detecting failure of fuel rods in a boiling water reactor. According to this method, the detection of a faulty fuel rod is achieved by using a cap system adapted to be mounted on the fuel assemblies. The cap system includes a cap portion having a plurality of caps each having a square cross-section and adapted to be mounted on the fuel assemblies, and water sampling tubes. Four water sampling tubes are arranged above the cap portion. Each water sampling tube is inserted into the corresponding cap. An air supply port is formed through the upper wall of each cap. Also, a handle is attached to the upper surface of the cap portion.
The check of the fuel assemblies is made in the following manner. As the operation of the boiling water reactor is stopped, the upper lid of the reactor pressure vessel is removed. Then, the handle of the cap system is grasped by the gripper of a fuel replacement device which is adapted to move the cap portion of the cap system to a position above the fuel assembly to be checked. Then, the lower end of the cap portion of the cap system is seated on an upper grid plate which supports the upper ends of the fuel assemblies in the reactor. Then, air is supplied into the cap portion through the air supply port. The supply of the air is ceased when the water level in the cap portion has come down below the upper end of the channel box. Thereafter, the cooling water in the fuel assembly is picked up by means of the water sampling tube and a sampling hose connected to the latter, and the level of radioactivity of the sample water is measured. The judgement as to whether there is any faulty fuel rod is made in accordance with the result of the measurement of the radioactivity level.
Thus, four fuel assemblies are checked by the cap system having four caps. The check of 400 to 800 fuel assemblies in the reactor core with this system, however, requires an impractically long time, which in turn constitutes one of the factors which hinder a prompt restart of the nuclear reactor, and thereby cause a lower plant factor for the nuclear reactor.